Numerous kinds of apparatus for detecting the position of something positioned in space are known.
Known methods to determine the direction of a beam of, for instance, electromagnetic energy often employ moving parts, which can be costly, complex and not always reliable. One example is the rotating antenna used in radar, in which the distance to a remote reflector is a calculated by measuring the time taken for the radar beam to travel from the transmitter to the reflector and back. Since the propagation speed is known, this time interval can be converted to a distance measurement. The actual position of the object can then be determined, the antenna direction at the time of measurement being known.
Many devices acting as transmitters and/or receivers of acoustic and electromagnetic energy have respectively an output power or input sensitivity which varies with the angular displacement relative to some datum direction in space. This property will be referred to as a polar characteristic. Examples of devices which exhibit a polar characteristic are radio or radar transmitting and receiving antennae, ultrasonic transducers, microphones, light emitting diodes (LED's), and photoelectric receivers. In many instances it is possible to control the polar characteristics of the device during manufacture.
Optical direction finding equipment is also well known in various forms, such as those used in navigation, surveying etc.
There is a need to establish the direction of an object from an observer who wishes to determine other factors such as the position of an object, or whether it is moving (and in that case, possibly also its direction of movement). Position finding apparatus is for example used in many forms in gunnery and navigation. Movement detectors are also important in security systems, for example for detecting an intruder.
Movement detectors, using the Doppler effect, are widely used in security systems to detect entry of intruders and initiate alarms or control other actions.
These detectors operate by transmitting energy of a specific wavelength into the space in their vicinity. The transmitted energy (or some of it) is reflected back, by objects in the space, to a receiver sensitive to this energy. The reflected signal is then mixed with a reference signal having identical wavelength to that of the transmitted energy. Any difference in frequency between the reflected signal and the reference signal causes low frequency amplitude modulation at the mixer output. If the reflecting objects in the vicinity of the device are stationary, then the transmitted and reflected signal frequencies are identical; but if the objects are moving there is a frequency differential between the received signals. This is proportional to the velocity of the moving objects (s), resulting in amplitude modulation of the mixed signal. By detecting the presence of this amplitude modulation it is therefore possible to determine that there is a moving object within the vicinity. This information may be used tot rigger an alarm sequence or any other desired control action. If the transmitter and receiver are located together, the speed of the object in a radial direction can also be measured by determining the frequency of the resultant amplitude modulation at the output of the mixer.
Since any number of energy waves, simultaneously present in the medium in which they propagate, will mix together to form an additive signal, it is possible to use this property as a ready=made mixing stage. It is therefore quite common in the design of movement detectors to supply the reference signal by causing a proportion of the transmitted energy to "leak" directly into the receiving device. The reflected energy then mixes with the reference signal and the receiver receives the resulting added signal. By demodulating (rectifying) the received signal, and passing this signal through a low-pass filter, any resultant amplitude modulation within the passband of the filter will be extracted from the composite received signal. This process is commonly described as "envelope following" or "demoudlation". The same technique is used in amplitude modulated (A.M.) radio reception, where the audio information is extracted from the amplitude component of the received signal.
In microwave motion detectors, the receiving diode receives and demodulates the microwave energy in one step, since its electrical output is only a function of the amplitude of the received microwave energy. This property obviates the need for rectification of the received signal and the subsequent low-pass filter stage.
The demodulated signal will contain a d.c. component corresponding to the average received signal amplitude. Since the effect of movement of any reflecting object in the vicinity of a detector is to introduce a.c. components to the demodulated signal, the d.c. component may be removed from the latter prior to any further signal analysis, without detriment to the intended function. The magnitude of the remaining a.c. component will be dependent on the angle of incidence of the received reflected signal and on its amplitude. It will be appreciated that the amplitude of the reflected signal at the point of reception will depend on the transmitted power, the reflectivity of the moving object, the surface area of its reflecting surface, and the distance and attenuation of the medium between the transmitter, the object, and the receiver.
Motion detectors currently used commonly suffer from false alarms brought about by, for instance, moving curtains or convection currents set up by central heating radiators etc.
The most popular current form of intruder detector is the passive infra-red motion (PIR) detector. These detectors respond to black body radiation from a human or other animate body, though they can be used to detect any moving object provided there is a temperature difference of at least 0.25.degree. C. between the object and its background or surroundings. These detectors use optical sensors, each comprising a pyroelectric sensing element in combination with suitable optical means to chop the incident radiation as the object moves.
These optical chopping has been achieved in various ways, one being by the use of a multi-faceted mirror with a pyroelectric sensor at the common focus of the facets. As the object moves across the field, it comes in and out of view, so modulating the incident radiation which causes the sensor to produce an appropriate output signal. Another example of a known chopping device is a Fresnel lens array (a series of lenses radially arranged with coincident focal points) with a pyroelectric sensor at the common focal point. Radiation from an object traversing the field passes through a number of the lenses with consequent chopping of the incident radiation.
Conventional pyroelectric detectors can also be activated by moving air currents, e.g. those associated with central heating, curtains or other harmless moving objects, and are also usually sensitive to incandescent lamp radiation if it has any significant modulation component.
Another type of detector system is a flame detecting system for locating the presence and the direction or position of a fire. Fires often occur out of doors in daylight. Under these circumstances, early detection of a fire may be very difficult where the flame is very small and the daylight is bright. Conventional fire detection systems often include a flicker frequency analyser facility which enables the system to discriminate between the flickering energy characteristic of a flame and other received energy such as sunlight.
Fire detection systems commonly use pyroelectric sensors. However, as is well known, pyroelectric sensors are prone to suffer from microphony effects and self-induced noise, which in currently-known systems seriously limit the range of distance within which detection is possible. At the same time, it is normal in flame detection systems to include a flicker frequency analyser (FFA) to compare the input from the sensor with a set of predetermined criteria for identifying the flame as such by detection of flicker.